<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>For 循环优化</title>
</head>
<body>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <p class="btn">add</p>
  <p class="btn">add</p>

  <script>
    var btns = document.getElementsByClassName('btn')
    for (var i = 0; i < btns.length; i++) {
      console.log(i)
    }

    for (var i = 0, len = btns.length; i < len; i++) {
      console.log(i)
    }
  </script>
</body>
</html>